The general problem of extending battery life of a remote communication unit has been recognized for a long time. This problem can be appreciated when considering that a remote communication unit, such as a portable radio or a pager, has a limited energy supply (i.e., a battery) for powering the communication unit. In the communication unit a receiver monitors a communication channel for receiving information and/or messages. If the receiver continuously monitors the channel, power consumption is increased, and the life of the energy supply is decreased.
Prior attempts in solving this problem have resulted in many and varied solutions. These attempted solutions have typically involved enabling and disabling a battery saving mean for periodic intervals such that the unit remains inactive (or "OFF") for most of the time, and becomes active (or "ON") at periodic short intervals to monitor a communication channel. Generally, during the OFF period, major portions of power consuming circuitry of the communication unit are inactivated leaving only a small timing circuit active. This timing circuit, at the end of a predetermined interval, activates the entire communication unit circuitry. If during the ON period an attempt to communicate with the unit, or a call, is discovered, the communication unit remains on for an extended period of time or until the call is terminated.
In a time division multiplexed (TDM) system the communication channel is divided into a predetermined number of time slots, wherein a remote communication unit may communicate with a central control station within an assigned time slot. For example, a radio may be assigned to a 100 ms time slot for transmission and reception of a message in a regularly reoccurring sequence of time slots. If so, the battery saving circuits of each individual radio may be designed so as to synchronously monitor centrally transmitted synchronization signals at their respective predetermined assigned time slots, but otherwise be in a substantially unenergized power saving mode of operation.
Some prior art circuits have also used an inverse control procedure where the receiver is turned "OFF" for predetermined intervals only if it is successfully receiving a specified digital synchronization pattern. Furthermore, there have been prior art attempts to divide the radios of a TDM system into predetermined groups that have special group addressing codes (e.g. tones, digital words, etc.). One such system is disclosed in U.S. Pat. No. 4,389,192 issued to the applicant of the instant application, which enhances the battery saving features by requiring identification of one of these predetermined group address signals before a given radio receiver or radio is removed from its standby battery saving mode of operation. Accordingly, when communication attempts are being made to a given radio, only the radios in that given group will be taken out of the battery saving mode of operation.
Some other prior art systems are asynchronous in operation. For example, all radios may have a regular sampling interval of 0.5 seconds and a group preamble tone (which precedes any attempt to communicate with one or more receivers of a given group), which is substantially longer than 0.5 second in duration. Accordingly, no matter when an attempt is made to communicate with a given group of stations after the initial preamble tone has ended, all radios of that given group will have been removed from their battery saving mode of operation, and energized so as to accept any communication addressed thereto or perhaps addressed generally to every radio of a given group.
Another approach for battery saving of remote radios is described in U.S. Pat. No. 4,449,248 issued to Leslie et al. Leslie et al. discloses a system wherein the duration of the ON/OFF ratio of a group of remote radios is programmed by by central station. In Leslie's system, the ON/OFF timing ratio is programmed into the communication unit, according to an information signal received from the control station. Accordingly, a programmable timer is programmed to activate and deactivate the power supply based on the received information signal. The communication with the radio occurs at intervals during which the corresponding group of radio's are active.
However, in a TDM system there is a need to provide a more effective power saving for the remote radios during situations where a substantially prolonged out-of-synchronization or lack of communication with the central control station is anticipated, such as when the remote radios are out of communication range with the central control station, when a call is in process between another unit and the central control station, and/or when the central control station is experiencing a malfunction.